1. Field of the Invention
The present invention relates to fault tolerance in computer systems, and more particularly to a system for removing power and signals from a bus card that is inadvertently removed from a computer system during a "hot swap" operation while the computer system is running.
2. Related Art
The demand for reliability in computer systems has led to the development of computer systems that support "hot swapping" of bus cards. This allows a computer system to continue operating while a bus card is removed from the computer system, and while a new bus card is inserted into the computer system. In this way, failed system components located on bus cards can be replaced without shutting down the computer system. The other bus cards in the computer system can continue to function while the defective bus card is being replaced.
Existing hot swapping systems typically use software routines to remove power from a bus slot before a bus card is removed from the bus slot. A similar software routine is used to "power up" a new bus card after the new bus card is inserted into the bus slot. These power removal and power up functions prevent the bus card and the computer system from being damaged when the bus card is removed and/or replaced.
However, hot swapping can lead to other problems. If a computer system operator is not careful during the hot swapping process, the computer system operator may inadvertently remove the wrong bus card. This is an easy mistake to make because bus cards are typically packed close together and are often similar or identical in appearance. In order to guard against inadvertent removal, the computer system operator must take special care to power down the proper bus card, and to properly identify the powered down bus card for removal.
Some existing systems provide light emitting diodes (LEDs) near each bus card to indicate whether or not the bus card is receiving power. By examining these diodes, the computer system operator can locate the proper card to remove. However, these LEDs are often a number of inches from their corresponding bus card connectors, and the bus cards are often packed very closely together. Consequently, an operator can still easily remove the wrong bus card during the hot swapping process, in spite of the presence of the LEDs.
Removal of the bus card while the computer system is operating can have serious consequences. At a minimum, the system is likely to "hang" while waiting on an uncompleted bus transaction involving the removed bus card. At worst, removal of the bus card from an operating computer system can cause damage to the computer system and/or the bus card.
What is needed is a mechanism that removes power and signals from a bus card before the bus card can be inadvertently removed from a computer system.